If by "resolving CNAMEs", you mean doing DNS resolution to
obtain A or MX records ...
No, I meant this part of RFC 1123:
5.2.2 Canonicalization: RFC-821 Section 3.1
The domain names that a Sender-SMTP sends in MAIL and RCPT
commands MUST have been "canonicalized," i.e., they must be
fully-qualified principal names or domain literals, not
nicknames or domain abbreviations. A canonicalized name either
identifies a host directly or is an MX name; it cannot be a
CNAME.
I think we all agree that was a mistake, and CNAMEs are fine in MAIL and
RCPT and you shouldn't mess with them, but I can't find any place this
advice has been explicitly deprecated.
R's,
John
PS: Even 5321 says the EHLO argument can't be a CNAME.
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